The Third Generation mobile cellular technology, also known as the Universal Mobile Telecommunications System (UMTS), is based on wideband code division multiple access (WCDMA) radio technology, which offers greater spectral efficiency and higher bandwidth than earlier systems. The radio access network part of a UMTS system is referred to as the UMTS Radio Access Network (UTRAN).
The Third Generation Partnership Project (3GPP) is responsible for the development of standards relating to WCDMA and UTRAN.
The High Speed Downlink Packet Access (HSDPA), a part of the WCDMA standard, has introduced a new downlink transport channel, potentially increasing download speeds to more than 10 Mbit/s. The uplink equivalent, High Speed Uplink Packet Access (HSUPA), is sometimes referred to as Enhanced Uplink (EUL). Together, these two technologies are known as High Speed Packet Access (HSPA). The performance increase provided by HSPA will further improve the end-user experience for web access, file download/upload, voice over IP (VoIP) and streaming services.
Similarly to many other communication systems, the processing in WCDMA is structured into different protocol layers, one of which is the Medium Access Control (MAC) layer The MAC layer provides logical channel multiplexing, hybrid-ARQ retransmissions, and uplink and downlink scheduling. The MAC layer may be divided into several entities, where each entity is responsible for handling transmission on one or more transport channels. For example, MAC-b is the MAC entity that handles the broadcast channel.
Hybrid-ARQ (Automatic Repeat Request), also known as HARQ, is a method for correcting transmission errors where data units that are not acknowledged by the receiver are automatically retransmitted. Forward error correction bits are also added to the data to enable the receiver to detect if a packet has been incorrectly received.
In layered protocol structures, the data units that a layer receives from the layer above it are generally referred to as Service Data Units, SDU:s. Within each protocol layer, some processing may be performed and layer-specific header information is added to the SDU:s, thereby forming Protocol Data Units, PDU:s, which are forwarded to the layer below. Thus, the PDU of one layer is the SDU of the layer below it.
The MAC layer receives its data from the Radio Link Control layer, RLC, which is the layer directly above MAC. Hence, the data units that MAC receives from RLC are referred to as MAC Service Data Units or MAC SDU:s. Within the MAC layer each SDU is processed and encapsulated, i.e. MAC-specific header information is added to the data. The resulting data units are referred to as MAC Protocol Data Units, or MAC PDU:s. A MAC PDU may contain more than one MAC SDU. Once MAC finishes its processing it forwards the PDU:s on to the Physical layer for further processing and transmission.
An RLC PDU (i.e. a MAC SDU) was previously of a fixed size. However, the HSDPA technology evolved further with the introduction of higher-order modulation and multi-antenna transmission, increasing the theoretical peak data bit rate to 21 Mbps. In order to sustain such high data bit rates, a large RLC PDU size is needed. Therefore, a new MAC entity, MAC enhanced high speed (MAC-ehs), was introduced. This entity is optimized for HSPA and supports flexible RLC PDU sizes in the downlink. In other words, the transmitter may select the size of the RLC PDU freely to make the best possible use of the available radio resources. However, radio conditions can sometimes change rapidly, so that by the time an RLC PDU is actually transmitted, it is too large to be sent over the air interface with a reasonable number of HARQ retransmissions. To address this problem, MAC-ehs also supports segmentation of RLC PDU:s in the downlink, i.e. large RLC PDU:s may be split up by MAC-ehs into several smaller segments which are transmitted separately, and then concatenated back together at the receiver side.
It is desirable for the MAC protocol to support flexible RLC PDU sizes and segmentation in the uplink direction as well. In the uplink another MAC entity, improved MAC (MAC-i/is), has therefore been introduced. The MAC-i/is header includes the following MAC fields, which generally are very similar to the fields already defined for HSDPA in downlink:                A LCH-ID field comprising an identifier of the logical channel that the data belongs to (size: 4 bits).        An L field, comprising a length indicator that indicates the length of an SDU or SDU segment in the MAC PDU (size: 11 bits).        F flag, indicating if another set of LCH-ID, L, and F fields follows, i.e. if more than one SDU is present in the MAC PDU (size: 1 bit)        SS, Segmentation status field, indicating how the MAC segmentation is performed (size: 2 bits)        A TSN field, comprising a transmission sequence number. The field is used to reorder the MAC PDU:s such that they are delivered to higher layers in sequence (size: 6 bits).        
Thus, the total MAC header size is 24 bits. Regardless of the exact format used, it is evident that the header contributes to a significant overhead, especially when the transmitted MAC PDU is small. Due to the introduction of flexible RLC PDU sizes and MAC segmentation in uplink, small MAC PDU:s are foreseen, especially at the cell edges where radio conditions are typically worse. The problem occurs in both the uplink and downlink directions, but is more severe in the uplink. This is because the UE has less transmission power available compared to the NodeB, and is therefore less capable of compensating for deteriorating radio conditions.
This increased overhead will result in a reduction of the coverage that the system can provide, e.g. measured as the distance from the base station that a certain bit rate can be supported, as a large part of the PDU will be occupied by header information instead of data.